1. Field of the Invention
The invention relates to multiphase direct current power supplies and refers more particularly to such power supplies wherein the power supply secondary circuit impedance is reduced through a Y connection of the primary circuit transformer primary windings whereby current through the secondary circuit of the power supply is split between multiple diode paths.
2. Description of the Prior Art
In the past, large three-phase direct current power supplies for example might have an open circuit, that is zero current output voltage, of approximately 19 volts. At 100,000 amperes, such output voltage may drop to approximately 8 volts. Thus, approximately 11 volts may be dropped inside such power supply.
In an effort to improve the efficiency of such power supplies, and on investigation it has been found that the power supply transformers have a 7% impedance factor. That is, the voltage output of the transformers drops approximately 7% from a no-load to a full-load condition. Using this factor, the transformers themselves contribute approximately 1.5 volts to the total internal voltage drop of such power supplies.
It is usual in such power supplies to have six sets of ten diodes each in parallel in the power supply secondary circuit. Such a circuit is referred to as a 6-phase star. With the usual connection of prior power supply circuits, one set of ten diodes is conducting the full output current at any time. Therefore, each diode of a set when conducting may conduct 10,000 amperes. Manufacturer's information for such diodes indicates that the voltage drop across such diodes increases from approximately 0.5 volts at low currents to 1.5 volts at 10,000 amps. Therefore, the voltage drop through the diodes accounts for approximately 1.5 volts of the total internal voltage drop of such three-phase direct current power supplies.
With such prior power supplies, the analysis of the copper cross section area of the internal buses of the power supply indicates that the voltage drop due to the resistance of the copper is normally less than 0.5 volts.
Thus, the voltage drop through the transformers of such power supplies, and the copper and diodes thereof, account for about 31/2 volts of the total 11 volt drop in such power supplies. Most of the remaining voltage drop comes from inductance. That is, the conductors carrying current from the transformers through the diodes to the output of the power supply have inductance. Since each diode set transitions from non-conducting to conducting, in the operation of the power supply, the inductance resists the flow of current.
Inductance in this area shows up as a non-unity power factor in the primary of the transformers. Additional open circuit transformer voltage is needed to overcome this inductance which results in a higher primary current demand for a given secondary current.
With reference to the prior art, connection of the primary circuit of three-phase direct current power supplies as considered above, areas are noted where power supply impedance may be improved.
Thus, due to the delta configuration of the transformer primary windings shown in FIG. 1, only one transformer winding at any instant of time is conducting. This means that only one set of diodes in the secondary circuit of the power supply will be conducting at any time. Therefore, the entire current of a power supply with transformer primary windings connected as shown in FIG. 1 has to flow through the impedance provided by one transformer, one diode set, and the associated conductors. Despite the fact that there are six possible current paths through the power supply, one through each diode set, there has been no use made of any possible parallel current paths to reduce the internal impedance of prior multiphase direct current power supplies.